Default sky lightning no longer illuminates terrain

For some reason, after saving my map and then loading it up again, the default scene light no longer illuminates the terrain. It’s completely flat shaded:

I didn’t change anything at all from when it was working. The scene itself is very simple, I used the blank game blueprint, imported my terrain heightmap, created a material with a grey base colour, and that was it. Saved, closed UE4, loaded it up after a while, and now the light does not affect the terrain at all.

Any idea why that is?

You’re in the Unlit viewmode, look at the top left of the viewport, second button from the right. Click that and change it back to “Lit”

Aaah! You’re a genius, thanks mate.